We have recently found that a cell line (SK-N-MC) of neuroectodermal origin is susceptible to productive HIV-l infection in vitro despite the lack of detectable CD4. Furthermore, the infection of this neuronal cell line cannot be blocked by anti-CD4 antibodies which are capable of preventing HIV-l infection of T- helper lymphocytes. These findings suggest that there is an alternate pathway by which HIV-l can enter and replicate in CD4- negative cells. We now propose additional studies to identify and characterize the HIV-l receptor on SK-N-MC cells. These neuronal cells will be cloned to select a subpopulation which is most susceptible to HIV-l infection. Studies will then be performed to examine the kinetics of HIV-l binding and infection in these cells, followed by quantitative analyses to determine the affinity of the interaction between HIV-l envelope glycoprotein (gp120) and SK-N- MC cells. Subsequently, identification of the putative second receptor for HIV-l will be pursued by ligand-receptor cross-linking experiments and by affinity chromatography. The identified receptor will then be used to generate polyclonal antisera and monoclonal antibodies, which will serve as probes to study the distribution of this receptor in human cell lines and tissues. In addition, we will molecularly clone the gene encoding the receptor by the rapid immunoselection technique of Seed and Aruffo using antireceptor antibodies, followed by sequencing of the receptor gene by the dideoxy-chain termination method. Lastly, we will attempt to determine the gp120 domain(s) which mediate binding and infection in SK-N-MC cells, since the necessary reagents to do so are already available to us. We believe that these studies to characterize a second receptor for HIV-l may reveal important information on the pathogenesis of AIDS or AIDS dementia.